1. Field of the Invention
The present invention relates to a video signal processing circuit for converting a digital component color television signal into a luminance signal and a carrier chrominance signal which can be received by a television receiver of NTSC system and, particularly, to a video signal processing circuit capable of easily converting a digital color difference signal into a carrier chrominance signal.
The MUSE-NTSC converter (referred to as M-N converter, hereinafter) is an example of a circuit which outputs a digital component video signal. The term "MUSE signal" means a high definition television signal encoded by the MUSE system which is one of the transmission standards for high definition television systems and is a video signal band compressing system developed by NHK (Japan Broadcasting Corporation. The M-N converter functions to decode the MUSE signal and convert it into the NTSC television signal. The M-N converter will be described as an example of the video signal processing circuit for converting the digital component color television signal into the luminance signal and the carrier chrominance signal which can be received by the conventional NTSC television receiver.
2. Description of the Prior Art
FIG. 3 is a block diagram of the conventional M-N converter. In FIG. 3, a MUSE signal is supplied to, an input terminal 1 and then to an A/D converter 2. The A/D converter 2 is supplied with a clock signal CK1 of 16.2 MHz outputted from a MUSE decoding processor 3 as a sampling clock signal. A digital MUSE signal outputted from the A/D converter 2, which is sampled at a rate of 16.2 MHz and quantized, is inputted to the MUSE decoding processor 3. Since the horizontal scanning frequency of the high definition television signal is 33.75 kHz, the digital MUSE signal sampled at the rate of 16.2 MHz has a data of 480 pixels per 1H (horizontal scanning period).
The MUSE decoding processor 3 decodes the input digital MUSE signal and outputs it as the high definition television signal whose video signal frequency band is restored. In this example, the sampling rate becomes 32.4 MHz by this decoding process and the number of pixels in 1H becomes 960. Further, the number of effective image pixels in 1H is 748 which is about 78% of 960 pixels.
The high definition television signal outputted from the MUSE decoding processor 3 is supplied to a scanning line converter 4 in which the horizontal scanning period and the number of scanning lines are converted, resulting a digital luminance signal Y and digital color difference signals R-Y and B-Y.
There are some conversion modes in an image conversion method for displaying the high definition television signal having an aspect ratio of 16:9 on a conventional television receiver having a screen having an aspect ratio of 4:3. Among them, a zoom mode for converting an image of aspect ratio of 16:9 into an image of aspect ratio of 4:3 by cutting away both side portions of the 16:9 image as shown in FIG. 4, will be described.
In the zoom mode, 1125 scanning lines of the high definition television signal is converted into 525 scanning lines which are 7/15 of the scanning lines of the high definition television signal.
In this example, 748 effective image pixels per 1H in the high definition television signal is converted into 560 pixels per 1H, which are 3/4 of 780 pixels, since 1/8 of the image is cut out on each side in the zoom mode.
Since, in the NTSC signal, the ratio of effective image pixels in 1H is about 84%, the total number of pixels in 1H is 668. Since the horizontal sync frequency of the NTSC signal is 15.75 kHz, a sampling clock signal CK2 for the luminance signal is 15.75 (kHz).times.668 (pixels)=10.52 MHz. In this example constructed as the M-N converter, the frequency range of color signal is a half of that of the luminance signal. Therefore, a frequency of sampling clock signal CK3 of the color difference signal is 5.26 MHz. A ratio of the clock signals CK1 to CK3 is 3600 to 1169. Based on the clock signal CK1 supplied from the MUSE decoding processor 3, the scanning line converter 4 generates the clock signals CK2 and CK3 by means of PLL circuit not shown.
In order to make the digital component video signal composed of Y, R-Y and B-Y which are outputted from the scanning line converter 4 receivable by the conventional television receiver, it must be converted into an analog luminance signal and an analog carrier chrominance signal.
In this view point, the digital luminance signal Y outputted from the scanning line converter 4 is converted into an analog luminance signal Y by a D/A converter 14 and the digital color difference signals R-Y and B-Y are converted into analog color difference signals R-Y and B-Y by D/A converters 15 and 16, respectively. In this case, a clock signal CK2 of 10.52 MHz is supplied from the scanning line converter 4 to the D/A converter 14 as a sampling clock signal and a clock signal CK3 of 5.26 MHz is supplied from the scanning line converter 4 to the D/A converters 15 and 16 as a sampling clock signal, for the reason mentioned previously.
The analog color difference signal R-Y is supplied to a multiplier 21 and the analog color difference signal B-Y is supplied to a multiplier 22. An oscillator 17 outputs a sine wave signal 18 having a color subcarrier frequency fsc (=3.579545 MHz). The sine wave signal 18 is supplied to the multiplier 22 and a phase advancer 19. The phase advancer 19 advances the phase of the sine wave signal 18 by 90 degrees with respect to the subcarrier of the color difference signal B-Y and a resulting sine wave signal 20 is supplied to the multiplier 21.
The multiplier 21 performs a balanced modulation of the color difference signal R-Y by the sine wave signal 20 and the multiplier 22 performs a balanced modulation of the color difference signal B-Y by the sine wave signal 18. The balance-modulated color difference signals outputted respectively from the multipliers 21 and 22 and having a phase difference of 90 degrees from each other are added to each other by an adder 23 and outputted therefrom as a carrier chrominance signal.
Thus, the conventional television receiver can receive the analog component video signal.
In the conventional video signal processing circuit, the digital component video signal is converted into an analog signal and, thereafter, analog color difference signals are converted into a carrier chrominance signal by an analog circuit, as mentioned before. Since it is difficult to arrange a digital system and an analog system in a single IC chip (integrated circuit) in a mixed state, the conventional video signal processing circuit is not suitable for a high density integration and it would be a source of increased cost.
Further, since an analog circuit has a relatively large number of adjusting points and is vulnerable to noise and interference, its operation is relatively unstable.